The invention relates generally to pulse width modulated power supplies, and deals more particularly with a pulse width modulated power supply having circuitry to protect a switch which pulse width modulates input voltage to a power train transformer.
A standard pulse width modulated power supply comprises a power train transformer having primary and secondary windings. One end of the primary winding is connected to a DC voltage source and the other end is connected to a semiconductor switch. The switch is repeatedly turned on and off to develop an AC voltage across the primary winding. A half or full wave rectifier bridge is connected across the secondary winding and charges an output capacitor to supply the output DC voltage. The following prior art technique is used to regulate the output voltage during start up and during normal operating conditions. A clock periodically sets an RS flip flop to turn on the switch to deliver power through the transformer. The output voltage is compared to a reference voltage and when the output voltage rises just above the reference voltage, a comparator resets the flip flop to turn off the switch. The switch stays off until the next clock cycle. The longer the switch stays on during each cycle, the higher the output voltage and vice verse. Thus, the feedback from the output voltage regulates the output voltage to the desired level. The reference voltage can be designed to increase gradually during start up so the output current is limited during start up to prevent large current surges. Afterwards, the reference voltage reaches a constant level so the power supply can deliver operating current. One problem with such a pulse width modulated power supply is that at initial start up, substantial current will still flow to the output capacitor of the power supply and a capacitive load and the switch will stay turned on for a long time. This may burn-out the switch.
Another prior art technique to control the switch uses a differential amplifier that receives at one input a fixed reference voltage and at the other input the output voltage. The output of the differential amplifier is called an "error voltage" and is proportional to the difference between the output voltage and the reference voltage. The output of the differential amplifier charges a reference capacitor through a current limiting resistor and diode. The voltage across the reference capacitor and diode forms a second reference voltage and is supplied to one input of a comparator. The other input is supplied by the voltage across a timing capacitor which is charged by rectified current from a secondary of another, sense transformer. When the voltage across the timing capacitor just exceeds the second reference the switch, the comparator resets the flip flop which shuts off the switch (until the next cycle of the clock). Also, the flip flop activates a transistor to discharge the timing capacitor. Because the second reference voltage rises gradually as the reference capacitor charges, the output current is limited during start up. Eventually, the reference capacitor charges to a fixed level and the timing capacitor requires sufficient time to rise to the fixed reference voltage so that the power supply can deliver operating current. The switch in this prior art power supply is also turned on for an extended time during start up because of the capacitive load and may burn-out.
Accordingly, a general object of the present invention is to provide a pulse width modulated power supply which avoids prolonged turn on of the primary winding semiconductor switch during start up and overload conditions.